I was down this road last month helping a friend with his sound system in his 71 P.U.
We put tweeters at ear level in the vertical panel of the cab "where the upper seat belt bolt is located". We installed "Mids" in the back panel of the cab below the rear window. The gas tank was relocated to the bed so we had all that room for the bass speaker. In the center of the cab on the back we made up a panel to hold the Amp and Capacitor "I think that's what it was called" and some other goodies. I wanted to install some 12" in the panel but it's not my truck.....he went with one of those Bazooka tubes that sits on the floor and placed it behind the seat.
Now on my 70 P.u. I just purchased some 3-way speakers that come in their own enclosure "box" and bolted them to the back of my seat and I was done
